Box File Format

In the past, boxes were just tar files of VirtualBox exports. With Vagrant supporting multiple providers and versioning now, box files are slightly more complicated.

Box files made for Vagrant 1.0.x (the VirtualBox export tar files) continue to work with Vagrant today. When Vagrant encounters one of these old boxes, it automatically updates it internally to the new format.

Today, there are three different components:

Box File - This is a compressed (tar, tar.gz, zip) file that is specific to a single provider and can contain anything. Vagrant core does not ever use the contents of this file. Instead, they are passed to the provider. Therefore, a VirtualBox box file has different contents from a VMware box file and so on.

Box Catalog Metadata - This is a JSON document (typically exchanged during interactions with HashiCorp's Vagrant Cloud) that specifies the name of the box, a description, available versions, available providers, and URLs to the actual box files (next component) for each provider and version. If this catalog metadata does not exist, a box file can still be added directly, but it will not support versioning and updating.

Box Information - This is a JSON documnent that can provide additional information about the box that displays when a user runs vagrant box list -i. More information is provided here.

The first two components are covered in more detail below.

Box File

The actual box file is the required portion for Vagrant. It is recommended you always use a metadata file alongside a box file, but direct box files are supported for legacy reasons in Vagrant.

Box files are compressed using tar, tar.gz, or zip. The contents of the archive can be anything, and is specific to each provider. Vagrant core itself only unpacks the boxes for use later.

Within the archive, Vagrant does expect a single file: metadata.json. This is a JSON file that is completely unrelated to the above box catalog metadata component; there is only one metadata.json per box file (inside the box file), whereas one catalog metadata JSON document can describe multiple versions of the same box, potentially spanning multiple providers.

metadata.json must contain at least the "provider" key with the provider the box is for. Vagrant uses this to verify the provider of the box. For example, if your box was for VirtualBox, the metadata.json would look like this:

{
"provider": "virtualbox"
}

If there is no metadata.json file or the file does not contain valid JSON with at least a "provider" key, then Vagrant will error when adding the box, because it cannot verify the provider.

Other keys/values may be added to the metadata without issue. The value of the metadata file is passed opaquely into Vagrant and plugins can make use of it. At this point, Vagrant core does not use any other keys in this file.

Box Metadata

The metadata is an optional component for a box (but highly recommended) that enables versioning, updating, multiple providers from a single file, and more.

You do not need to manually make the metadata. If you have an account with HashiCorp's Vagrant Cloud, you can create boxes there, and HashiCorp's Vagrant Cloud automatically creates the metadata for you. The format is still documented here.

As you can see, the JSON document can describe multiple versions of a box, multiple providers, and can add/remove providers in different versions.

This JSON file can be passed directly to vagrant box add from the local filesystem using a file path or via a URL, and Vagrant will install the proper version of the box. In this case, the value for the url key in the JSON can also be a file path. If multiple providers are available, Vagrant will ask what provider you want to use.